Angular 2 - 没有提供程序错误(即使我添加了提供程序)

我正在尝试从特定类调用服务,但由于某种原因它无法调用,即使我能够在其他类中使用它.我正在使用@Input.我不知道这是否是造成问题的原因.

码:

import { Component, Input, OnInit } from '@angular/core';

import { Category } from './../Category';
import { CertService } from './../../shared/Service/cert.service';

@Component({
  selector: 'app-cert-item',
  templateUrl: './cert-item.component.html'
})
export class CertItemComponent implements OnInit {

  @Input() category: Category;
  @Input() categoryId: number;
  constructor(
    private certService: CertService //Console error "No provider for CertService!"
  ) { }

  ngOnInit() {
    console.log(this.certService.getCategories());
  }

}  

app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { HttpModule } from '@angular/http';
import { routing } from './app.routing';
import { ReactiveFormsModule } from '@angular/forms';
import { NgbModule } from '@ng-bootstrap/ng-bootstrap';

import { AppComponent } from './app.component';
import { HeaderComponent } from './shared/header.component';
import { DropdownDirective } from './dropdown.directive';
import { CertsComponent } from './certs/certs.component';
import { CertItemComponent } from './certs/cert-category/cert-item.component';

import { CertService } from './shared/service/cert.service';
import { HttpService } from './shared/Service/http.service';

import { LoginComponent } from './login/login.component';


@NgModule({
  declarations: [
    AppComponent,
    HeaderComponent,
    DropdownDirective,
    CertsComponent,
    CertItemComponent,
    LoginComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
    HttpModule,
    routing,
    ReactiveFormsModule,
    NgbModule.forRoot()
  ],
  providers: [
    CertService, 
    HttpService
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

我试图调用的服务类(示例)

import { Injectable } from '@angular/core';
import { Http, Response, Headers } from "@angular/http";
import { Observable } from "rxjs/Rx";
import 'rxjs/Rx';

@Injectable()
export class CertService {

  constructor(private http: Http) { }

  getCategories() {
    return this.http.get('api/categories')
   .map((response: Response) => response.json());
  }

}

我也无法使用当前路线

this.subscription = this.route.params.subscribe(
      (params) => {...

...});

同样,上面的代码在其他类中工作正常但在此类中不行.

如果作为提供者提供了错误,则会发生这种情况.并且不太可能存在其他原因.

它在一个地方...shared/service/cert.service,而在另一个地方...shared/Service/cert.service.案件很重要.这取决于环境在构建过程中这些模块会发生什么,但这可能会导致错误,例如它们可能会重复并引用不同的对象.

要快速调试它,CertService可以从两个文件中公开window.CertService1,window.CertService2并在控制台中进行比较,如果两者都定义相同.